32 LILLIE. [Vol. X. 



cleavages. For a general survey the dotted vertical lines can 

 be left out of account, and the cleavages within one column 

 thought of as synchronous. I include in an appendix the 

 results of some of the most important works on cytogeny 

 reduced to tabular form. To make comparisons more simple 

 all systems of nomenclature have been reduced to the one 

 employed here; but the original system has, in most cases, 

 been included in brackets, after the common system. 



(g) General Remarks on the Cleavage and Germ-layers. 



Increasing accuracy and detail in the study of the cleavage 

 of the ovum has been one of the most marked tendencies of 

 recent years in embryology. Primarily undertaken, in the 

 Invertebrata at least, to explain conflicting statements as to 

 the origin of the germinal layers in general and the mesoblast 

 in particular, the study of the cleavage is leading to new ideas 

 on the promorphological state of the ovum, and the real nature 

 of differentiation. In the Polyclada, Annelida, and Mollusca, 

 not only the germinal layers, but systems of organs, and even 

 single organs have been traced back to their parent blasto- 

 meres, and it has been shown that cells of the same lineage, 

 even in widely separated species, undergo, as a rule, the same 

 ultimate differentiation. 



The mesoblast was the first of the germ-layers to be traced 

 back to a single cell. Kowalevsky (No. 52) in 1871 showed that 

 the mesoblast of Lumbricus could be traced back to two 

 posterior pole cells, derived from the entoderm, which budded 

 off anteriorly, a large number of small cells thus forming a 

 mesoblastic germ band. He postulated a double source for 

 the mesoderm of Euaxes : the larger part came from two large 

 cells derived from the posterior macromere ; the rest was 

 derived from two small cells, derivatives of the two lateral 

 macromeres. Rabl (No. 25) formulated his results in 1876 as 

 follows : " Das mittlere Keimblatt entsteht also nach unseren 

 Auseinandersetzungen aus zwei, am Mundrande der Gastrula 

 gelegenen Zellen, deren Verwandschaft zu den Zellen des 

 inneren Blattes eine viel innigere ist, als zu jenen des aus- 



